Field of the Invention
This invention relates to a capacitive touch panel, especially to a touch sensing circuit of the capacitive touch panel.
Description of the Related Art
Please refer to FIG. 1. FIG. 1 illustrates a schematic diagram of the conventional touch sensing circuit of a capacitive touch panel. Since the capacitive touch panel TP is easily interfered by display panel and power, as shown in FIG. 1, the input signal generator P inputs a sine-wave input signal TX having a frequency nf0 to a horizontal electrode E of the capacitive touch panel TP. A time function TX(t) of the sine-wave input signal TX can be expressed as:TX(t)=−Vref*sin(2πnf0t)  (1)
Wherein, Vref is a reference voltage; nf0 is the frequency of the sine-wave input signal TX; t is time.
Then, the sine-wave input signal TX is received and amplified by the parasitic mutual capacitance Cm at intersection points of horizontal electrodes E and vertical electrodes D of the capacitive touch panel TP and the analog front-end circuit AFE, and then outputted to the mixer circuit MIX.
Wherein, the time function VAFE(t) of the sine-wave voltage signal VAFE outputted by the analog front-end circuit AFE can be expressed as:
                              VAFE          ⁡                      (            t            )                          =                  Vref          *                      sin            ⁡                          (                              2                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                                  nf                  0                                ⁢                t                            )                                *                      Cm            Cf                                              (        2        )            
Then, the mixer circuit MIX will mix the sine-wave voltage signal VAFE outputted by the analog front-end circuit AFE and another sine-wave S having the same frequency nf0 into a mixed signal VMIX and output the mixed signal VMIX to the integrator circuit IN.
Wherein, the time function VMIX(t) of the mixed signal VMIX outputted by the mixer circuit MIX can be expressed as:
                                                                        VMIX                ⁡                                  (                  t                  )                                            =                            ⁢                              Vref                *                                  sin                  ⁡                                      (                                          2                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                                              nf                        0                                            ⁢                      t                                        )                                                  *                                  Cm                  Cf                                *                                  sin                  ⁡                                      (                                          2                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                                              nf                        0                                            ⁢                      t                                        )                                                                                                                          =                            ⁢                              Vref                *                                  Cm                  Cf                                *                                  sin                  ⁡                                      (                                          2                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                                              nf                        0                                            ⁢                      t                                        )                                                  ⁢                                  sin                  ⁡                                      (                                          2                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                                              nf                        0                                            ⁢                      t                                        )                                                                                                                          =                            ⁢                              Vref                *                                  Cm                  Cf                                *                                                      1                    -                                          cos                      ⁡                                              (                                                  2                          *                          2                          ⁢                                                                                                          ⁢                          π                          ⁢                                                                                                          ⁢                                                      nf                            0                                                    ⁢                          t                                                )                                                                              2                                                                                        (        3        )            
When the integrator circuit IN receives the mixed signal VMIX outputted by the mixer circuit MIX, the integrator circuit IN will integrate the mixed signal VMIX into an output signal Vout. If the integration cycle used by the integrator circuit IN to integrate the mixed signal VMIX is 1/nf0, then the time function Vout(t) of the output signal Vout generated by the integrator circuit IN can be expressed as:
                                                                        Vout                ⁡                                  (                  t                  )                                            =                            ⁢                                                ∫                  0                                      1                    /                                          nf                      0                                                                      ⁢                                  Vref                  *                                      Cm                    Cf                                    *                                      (                                                                  1                        -                                                  cos                          ⁡                                                      (                                                          2                              *                              2                              ⁢                                                                                                                          ⁢                              π                              ⁢                                                                                                                          ⁢                                                              nf                                0                                                            ⁢                              t                                                        )                                                                                              2                                        )                                    ⁢                  d                  ⁢                                                                          ⁢                  t                                                                                                        =                            ⁢                                                                    Vref                                          nf                      0                                                        *                                      Cm                                          2                      ⁢                                                                                          ⁢                      Cf                                                                      +                                  Vref                  *                                      Cm                    Cf                                    *                                      (                                                                  ∫                        0                                                  1                          /                                                      nf                            0                                                                                              ⁢                                                                        (                                                                                    -                                                              cos                                (                                                                  2                                  *                                  2                                  ⁢                                                                                                                                          ⁢                                  π                                  ⁢                                                                                                                                          ⁢                                                                      nf                                    0                                                                    ⁢                                  t                                                                                                                      2                                                    )                                                ⁢                        d                        ⁢                                                                                                  ⁢                        t                                                              )                                                                                                                          =                            ⁢                                                                    Vref                                          nf                      0                                                        *                                      Cm                                          2                      ⁢                      Cf                                                                      +                                  Vref                  *                                      Cm                    Cf                                    *                                      (                                                                  -                                                  sin                          ⁡                                                      (                                                          2                              *                              2                              ⁢                                                                                                                          ⁢                              π                              ⁢                                                                                                                          ⁢                                                                                                nf                                  0                                                                ⁡                                                                  (                                                                      1                                    /                                                                          nf                                      0                                                                                                        )                                                                                                                      )                                                                                                                      2                        *                        2                        *                        2                        ⁢                                                                                                  ⁢                        π                        ⁢                                                                                                  ⁢                                                  nf                          0                                                                                      )                                                                                                                          =                            ⁢                                                                    Vref                                          nf                      0                                                        *                                      Cm                                          2                      ⁢                      Cf                                                                      +                                  Vref                  *                                      Cm                    Cf                                    *                                      (                                                                  -                                                  sin                          ⁡                                                      (                                                          2                              *                              2                              ⁢                                                                                                                          ⁢                              π                                                        )                                                                                                                      2                        *                        2                        *                        2                        ⁢                                                                                                  ⁢                        π                        ⁢                                                                                                  ⁢                                                  nf                          0                                                                                      )                                                                                                                          =                            ⁢                                                Vref                                      nf                    0                                                  *                                  Cm                                      2                    ⁢                    Cf                                                                                                          (        4        )            
From the equation (4), it can be found that the output signal Vout generated by the integrator circuit IN is a DC voltage without any AC voltage. Therefore, it is easy for the analog-to-digital converting circuit ADC to convert the output signal Vout into digital values, so that touch point determination will become simpler. In addition, since the value of the output signal Vout is proportional to the parasitic mutual capacitance Cm at the intersection point of horizontal electrodes E and vertical electrodes D of the capacitive touch panel TP, the value of output signal Vout can be used to determine whether the intersection point of horizontal electrodes E and vertical electrodes D of the capacitive touch panel TP is touched or not.
With this capacitive touch sensing structure, the noises having a frequency N times of nf0 (N>1) can be totally filtered, and other noises can be also reduced accordingly. However, the drawback of the prior art is that, in order to filter the noises having lower frequencies, smaller frequency nf0 will be used and the integration cycle 1/nf0 used by the integration circuit IN will become longer; therefore, the capacitive touch sensing efficiency will become poor, even the frame report rate of the capacitive touch panel will fail to meet the product specification.
Therefore, the invention provides a touch sensing circuit of a capacitive touch panel to solve the above-mentioned problems.